I build cells using this
http://www.modthesims.info/d/420376 and I put Sims in it to store them in my secret slave dungeon so my prostitute Sim can woohoo with them to get infinite money if they have it. (Using this to do that
http://nraas.wikispaces.com/Woohooer ) The MTS link has jail furniture like wooden beds, jail bars, and a wall decoration.
There is also this mod:
http://www.simlogical.com/ContentUp...e/uploads/1892/ which has various links in it. I haven't tried it out but it seems to have some furniture with it as well.
You could browse through this for prison clothing
http://modthesims.info/t/477718
Here is a type of prison lot but it has store content with it so eh.
http://www.modthesims.info/download.php?t=530087
This one seems better but still requires some expansions:
http://www.modthesims.info/download.php?t=490094
This one has a plethora of deco items to play with:
http://simswiki.info/wiki.php?title..._Public_Service
There are also some The Sims Resource things that come up but I don't go there so I could be missing something fun.
Hope this helps though! :D